Electronic apparatuses, such as a cellular phones, smart phones, and tablet PCs, have traditionally employed an antenna module for RFID (Radio Frequency Identification) to provide the function of short-distance contactless communication.
This antenna module communicates by making use of inductive coupling to an antenna coil incorporated into a transmitter, such as a reader/writer. In other words, this antenna device can drive IC that converts a magnetic field which an antenna coil receives from the reader/writer into electric power, thereby functioning as a communication processor.
For reliable communication, the antenna module needs to receive at least a certain value of magnetic flux from the reader/writer via the antenna coil. Therefore, an antenna device according to a known example is provided with a loop coil in the cellular phone and receives magnetic flux from the reader/writer via this coli. 
In the antenna module incorporated into an electronic apparatus such as a cellular phone, metal such as a substrate in the apparatus, a battery pack, or the like receives the magnetic field from a reader/writer, thereby generating eddy current that repels the magnetic flux from the reader/writer. For example, considering the casing surface of a cellular phone, the magnetic field coming from the reader/writer tends to strengthen at the periphery portion of the casing surface and weaken near the center of the casing surface.
In the case of an antenna using the regular loop coil, the opening of the loop coil is positioned in the central portion of the cellular phone, where there is little reception of the above-described magnetic field that passes through the periphery portion of the casing surface. Therefore, with an antenna using a regular loop coil, the efficiency of receiving a magnetic field is poor.
Therefore, an Antenna device in which the loop antenna is disposed in the peripheral portion of the casing surface, where the magnetic field from the reader/writer is strong, and an antenna device that uses a magnetic sheet to increase magnetic flux and enhance performance have been proposed. In these antenna devices, the loop antenna is shaped to be rectangular, with the long sides disposed along the outer peripheral edge of the casing surface (for example, see PTL 1-3).